Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust passage to an engine intake passage. By controlling EGR to provide a desired engine dilution, engine pumping work, engine knock, as well as NOx emissions may be reduced. For example, at partial throttle operating conditions, providing EGR to the cylinders of the engine allows for the throttle to be opened to a greater extent for the same engine load. By reducing throttling of the engine, pumping losses may be reduced, thus improving fuel efficiency. Further, by providing EGR to the engine, combustion temperatures may be reduced (especially in implementations where EGR is cooled prior to being provided to the cylinders). Cooler combustion temperatures provide engine knock resistance, and thus increase engine thermal efficiency. Further still, EGR reduces a combustion flame temperature that reduces an amount of NOx generated during combustion.
In some approaches, gas exhausted from only one or more of a subset of cylinders may be recirculated to provide EGR to all cylinders of the engine. For example, an EGR conduit may be coupled to an exhaust of a dedicated EGR cylinder so that exhaust from the dedicated cylinder is introduced into the intake manifold of the engine to provide EGR. In this way, a substantially fixed amount of EGR flow may be provided to the engine intake.
In such approaches which use dedicated EGR cylinders to provide EGR to the engine, the inventors herein have recognized that it may be desirable to run the dedicated EGR cylinder rich to increase ignitability of the air, fuel, EGR mixture. The ignitability may be improved due to the presence of hydrogen which is formed in the dedicated cylinder when running rich. Overly increasing the amount of fuel injected into the dedicated cylinder may lead to reduced combustion efficiency and/or increased smoke conditions during engine operation. For example, increasing richness in the EGR cylinder beyond that required for best combustion efficiency may cause smoke formation, and further increasing richness may reduce the ability to ignite the charge. As such, the amount of fuel that can be added to a dedicated EGR cylinder may be limited.
Thus, in one example, some of the above issues may be at least partly addressed by a method comprising, prior to combustion, injecting a first amount of fuel to a dedicated EGR cylinder, e.g., in an amount that provides an optimal combustion efficiency, and after combustion and during an expansion and/or exhaust stroke, directly injecting a second amount of fuel to the dedicated EGR cylinder. The first and second injections may be during a common cylinder combustion cycle, and may be repeatedly performed in successive cycles of the dedicated EGR cylinder.
In this way, an increased amount of fuel may be introduced into the EGR flow while maintaining good combustion with low soot formation. Further, in such an approach, pumping work at part-throttle for the remaining cylinders in the engine may be reduced via fuel evaporation in the dedicated EGR cylinder and fuel injectors in the remaining cylinders may be downsized resulting in cost savings and increased fuel efficiency. Further still, such an approach may be employed during engine cold start conditions while operating the dedicated cylinder in a lean mode when less than full EGR is desired. For example, to help with fuel vaporization, a small amount of fuel could be burned (via a stratified charge injection a during compression stroke of the dedicated EGR cylinder) to heat the air/cylinder and then fuel could be injected later in the cycle to improve evaporation of the fuel. In this way, fuel preparation, e.g., smoke reduction in direct injection applications, during warm-up of the engine may be improved.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.